Electronic circuit for exciting asynchronous-start slipringless synchronous motors

ABSTRACT

In order to excite the field winding on the rotor of an asynchronous-start synchronous motor which has a resistor connected in parallel with the winding, the alternating current output from an exciter winding mounted on the rotor shaft is converted in a rectifier bridge of the semiconductor type also mounted on the rotor shaft and fed to the field winding through a direct current circuit which includes a thyristor for interrupting the flow of direct current to the field winding when the motor is operating in its asynchronous state. For this purpose, a pair of series-connected resistors extend between the positive side of the rectifier bridge and the control electrode of the thyristor, and a diode has its anode side connected to the junction between the series-connected resistors and its cathode side connected to the negative side of the rectifier bridge. The thyristor for interrupting current flow from the rectifier bridge to the field winding can either be an auxiliary thyristor unit, in which case the semiconductor elements of the rectifier bridge can consist entirely of uncontrolled diodes, or the function of the thyristor in disconnecting flow of direct current to the field winding can be performed by so constructing the rectifier bridge that uncontrolled diodes make up the positive half of the bridge and thyristors the negative half.

United States Patent [191 I Canay ELECTRONIC CIRCUIT FOR EXCITINGASYNCHRONOUS-START SLIPRINGLESS SYNCIIRONOUS MOTORS [75] Inventor:Muzaffer Canay, Birr, Switzerland [73] Assignee: Brown B'overi & CompanyLimited,

Baden, Switzerland [22] Filed: May 22, 1973 2211 Appl. No.: 362,775

[30] Foreign Application Priority Data Merhof et a1. 318/193 X PrimaryExaminer-Gene Rubinson Attorney, Agent, or Firm-Pierce, Scheffler &Parker [451 Feb. 19, 1974 [5 7] ABSTRACT mounted on the rotor shaft isconverted in a rectifier,

bridge of the semiconductor type also mountedon the rotor shaft and fedto the field winding through a direct current circuit which includes athyristor for interrupting the flow of direct current to the fieldwinding when the motor is operating in its asynchronous state. For thispurpose, a pair of series-connected resistors extend between thepositive side of the rectifier bridge and the control electrode of thethyristor, and a diode has its anode side connected to the junctionbetween the series-connected resistors and its cathode side connected tothe negative side of the rectifier bridge. Thethyristor for interruptingcurrent flow from the rectifier bridge to the field winding can eitherbe an auxiliary thyristor unit, in which case the semiconductor elementsof the rectifier bridge can consist entirely of uncontrolled diodes, orthe function of the thyristor in disconnecting flow of direct current tothe field winding can be performed by so constructing the rectifierbridge that uncontrolled diodes make up the positive half of the bridgeand thyristors the negative half.

8 Claims, 2 Drawing Figures ASYNCI'IRONOUS-START SLIPRINGLESSSYNCIIRONOUS MOTORS This invention relates to an improvement in a knownelectronic circuit for exciting asynchronous-start slipringlesssynchronous motors, wherein the rotating winding, of an alternatingcurrent exciter armature mounted on the shaft of the synchronous motoris connected by way of a co-rotating rectifier bridge connection to therotating field winding provided in the synchronous motor and having atleast one parallel resistor, and wherein controlled converter valves areprovided for the temporary separation of the rectifier bridge connectionfrom thefield winding in the asynchronous state.

In a known circuit arrangement possessing these basic features theco-rotating rectifier bridge circuit is in the form of a semi-controlledbridge with thyristors in one half of the bridge and uncontrolled diodesin the other half. For the control of the thyristors, a control deviceis provided which is likewise fed by the armature winding of theexciterandwhich consists of a second rectifier bridge connection havingonly uncontrolled diodes. The control device is fed from the armaturewinding by way of an isolating transformer. Here the starting resistor,which; is connected in parallel to the field winding, always remainsconnected even in normal synchronous operation, and thus gives rise to 1increased excitation losses. During asynchronous starting, thethyristors remain blocked, so that the entire starting current in thefield circuit flows through this resistor in both directions.

In another known arrangement, the rectifier bridgecircuit comprisesuncontrolled diodes. During normal synchronous operations, the currentcoming from the diode bridge flows through a main thyristor, which isthen fired. 'Two auxiliary thyristors, which are connected in series inthe same direction and which are not fired in normal operation, .areconnected in antiparallel to the diode bridge. During asynchronousstarting, these auxiliary thyristors nevertheless become conductive, sothat the magnet wheel '(rotor) current can flow in one direction'throughthe diode bridge and in the opposite direction through the auxiliarythyristors. On the other hand, the main thyristor cannot fire withoutexcitation during asynchronous starting, and therefore a startingresistor in parallel to it comes into action in the field circuit duringstarting. All three thyristors are power thyristors.

Three power transistors are also contained in a third known circuitarrangement having an uncontrolled diode bridge, namely a main thyristorfor rated exciter current from the diode bridge to the field winding andtwo anti-parallel auxiliary thyristors, connected in series with aresistor, in a branch in parallel to the field winding. All three powerthyristors are controlled by a co-rotating control unit.

The above-mentioned circuits have certain disadvantages of which, in theease of the last-mentioned arrangement, only the complicated controlunit will be mentioned. In addition, not only in this arrangement butalso in the second circuit arrangement three power thyristors arenecessary, while finally in both circuits, the switching-on of theexcitation in dependence on the position or polarity of the rotor isnecessary, that is to say provision must be made for special monitoringof the magnet wheel (rotor) current. In the second circuit arrangementdescribed'above,moreover, a relatively high permanent grid current flowsin the main thyristor during normal operationsAlthough the arrangementmentioned first above permits higher powers than the other twoarrangements, and although it permits the switching-on of the excitationindependently of the position of the rotor, nevertheless it requires acontrol device consisting of a diode reactifier bridge and also a fairlyexpensive isolating transformer.

The invention seeks to avoid the disadvantages of theknown'arrangements. According to the invention this is' achieved in anelectronic excitation circuit arrangement of the type first describedabove by disposing on the negative side of the rectifier bridgeconnection, and in the direct current circuit of the latter, theanodecathode section which in relation to the rectifiers isforward-biased of at least one thyristor, one pole of which is connectedto one terminal of the parallel resistor, while the other parallelresistor terminal, which is connected to the rectifier bridge pole noton the thyristor side, is connected, throughthe series connection of tworesistors, to the control electrode of the aforesaid thyristor -at leastone such thyristor being provided, while the interconnection point ofthe two series resistors is connected by way of a forward-biased diodeto the anode of this thyristor.

The invention is explained more fully below with the aid of FIGS. 1 and2, in which:

, FIG. 1 shows an example of embodiment with an uncontrolled diodebridge, and

FIG. 2 a further development with a self-switching thyristor-diodebridge.

In'the circuit arrangement illustrated in FIG. I the rectifier bridge 1,which is in the form of a conventional uncontrolled diode bridge, isconnected in series with a thyristor 2. 3 is the resistor which is knownper se and is in parallel to the field winding'4, and 5 the threephasearmature winding of the exciter. From the positive pole J of the diodebridge '1 a connection is made by way of the series resistors 6 and 7and of the forward-biased diode 9 to the control electrode (the grid) ofthe thyristor 2.

In normal synchronous operation, during which the potential of thebridge pole] is higher than that at K,

at the cathode side of thyristor 2 a grid current flows through thefiring resistors 6 and 7, so that the thyristor 2 fires and becomesconductive. Resistors 6 and 7 have a very high resistance value inrelation to that of the starting resistor 3. According to the sameprinciple, that is to say, by means of a firing resistor, the mainthyristor in the known circuit arrangement, of the second kind discussedpreviously, is also fired. After the firing, I

I voltage. According to the invention the interconnection point of thefiring resistors 6 and 7 is now connected by way of the previouslymentioned forwardbiased diode 8 to the anode-of the thyristor 2. Withthe thyristor 2 non-conductive, resistor 7 is then fully effective. Assoon as thyristor 2 fires and becomes conductive, the diode 8 howeverpractically short-circuits this resistor 7. Consequently, the gridcurrent is reduced to a minimum, in accordance with the approximatelyconstant voltage drop at this diode. This very low permanent gridcurrent consequently does not vary to any considerable extent when theexciter voltage is raised.

During asynchronous starting, the field current flows during thenegative half-wave (potential of .l higher than that at A at the anodeside of thyristor 2) through the starting resistor 3, because of theblocking by the diode bridge. During the positive half-wave (potentialat A higher than that at J) thyristor 2 remains blocked, since itreceives no grid current at all. Thus, the current induced in the fieldwinding 4 always flows through the resistor 3. As soon as the motor isexcited (that is to say the potential at J is higher than that at K),thyristor 2 becomes conductive and the motor becomes synchronized. Theexcitation then automatically builds up only during the positivehalf-wave, even if it is switched on during the negative half-wave. Theexcitation can therefore be switched on at any desired time and at anyarbitrary slip value, and no relay is needed to pick up the positivehalf-wave, as is the case in the second and third solutions discussedabove. The proposed circuit arrangement according to FIG. Ithereforedoes not differ in respect of its behavior from conventional synchronousmotors but, in contrast to the latter, requires no sliprings or brushes.

In relation to the second and third known circuit arrangements describedabove, the arrangement according to FIG. 1 has the additional advantageof relatively simple construction with only one power thyristor (whereasin the known circuits referred to, two thyristors are additionallyrequired for one and the same motor power).

The second diode 9 in the grid circuit is not of primary importance forthe correct functioning of the circuit arrangement. It is, however,advisable in'practice, in order to avoid undesired. negative gridcurrents (anode-gr'id), which sometimes occur at high slip values, whenthe thyristor 2 is conductive.

A certain disadvantage of the circuit arrangement shown in FlG. l isthat, as in the case of the second and third arrangements previouslydiscussed, its applicability depends on the power of the thyristor.Adaptation to higher motor power is, however, possible if, in accordance with a further development of the invention, a pluralityofthyristors', for example three, are connected with their anode-cathodesection in parallel and arecontrolled conjointly. in this arrangementthere is only one first diode 8, which lies between the connection pointof the resistors 6, 7 and the interconnected anodes of the parallelthyristors. When diodes 9 are used between the firing resistor 7 and thethyristor grid, a separate diode 9 should be provided for each of theparallel thyristors.

In a further development of the invention, the thyristors mayso-to-speak be integrated" in the rectifier bridge 1, that is to saysome of the uncontrolled diodes of the latter, particularly the diodesof the negative half of the bridge, may be replaced by these thyristors2. The resulting circuit arrangement with a selfswitchingthyristor-diode bridge is illustrated in FIG. 2. Here again only asingle diode 8, which is common to'all three thyristors, is requiredbetween the connection point of the resistors and the negative pole ofthe bridge, which corresponds to the interconnection point of the threethyristor anodes. The other terminal of the second firing resistor ishere preferably connected by way of separate small diodes 9' similar tothe diode 9 in FIG. l, to the control electrode ofeach thyristor 2'.Thus, the thyristors can, without hindrance, apply their full blockingvoltage during the blocking period. With very low alternating voltagesof the exciter, it is, how over, here again possible to dispense withthese diodes,

In connection with the mode of operation of the arrangement shown inFIG. 2, it is pointed out that, as soon as the synchronous machine isexcited, the thyristor 2 whose cathode has the lowest negative potentialwill fire first, because the grid potential is already positive in allthe thyristors. After the firing of the first thyristor, the common gridcircuit is brought by the diode 8' to a potential which, because of thepractically constant voltage drop of the diode 8', is about ll volthigher than the common anode potential. Apart from this difference of 1volt, the grids and the anodes thus have the same potential. If thecathode potential V, of a thyristor 2, which is not yet conductive, islower than the common anode potential V this thyristor willautomatically fire. ln normal synchronous operation with excitation, thethyristors in thisarrangement are selffiring, that is to say they behaveas diodes. The bridge therefore does not differ from the conventionaldiode bridge.

During asynchronous starting, the pole wheel induced field current isblocked by the thyristors 2' during the negative half-wave and flowsthrough the resistor 3'. During the positive half-wave, the thyristors2' are unable to fire until synchronization, that is to say untilexcitation, is introduced, because the diode 8' is blocked during thishalf-wave, that is to say it has a resistance which is also very high inrelation to that of the firing resistors 6', 7. The grid potentialaccordingly corresponds substantially to the potential, which at thismoment is negative, of the terminal of field winding 4' connected to thecathodes of the uncontrolleddiodes 10 of the rectifier bridge 1. Thus,no grid current flows and the bridge remains blocked.

In the circuit arrangements proposed according to the invention, thepossible dispersion of the thyristor firing characteristics presents nodifficulty. The dimensioning of the firing resistors 6,7 presents noproblem since they have only protective functions. They 'do not need tobe adapted to each machine.

In the circuit arrangements illustrated in the drawings, the thyristorsnaturally do not receive any grid current at all in the blocking phase.This is a great advantage over externally controlled thyristor circuitar rangements in which extremely disagreeable and disturbing currents orsignals may occur when the signals arrive during the blocking period inconsequence of an error.

I claim:

l. in an arrangement for exciting the rotatable field winding of anasynchronous-startsynchronous motor, the combination comprising anexciter having an alternating current output winding mounted on therotor shaft of said synchronous motor for rotation therewith,

a resistor connector in parallel with said field winding.

a rectifier bridge of the semiconductor type also mounted on said rotorshaft for rectifying the alternating current output of said exciter anddelivering direct current to said parallel-connected field winding andresistor, thyristor means connected into the negative side of saidrectifier bridge for interrupting the flow of direct current to saidfield winding when said motor is operating'in its asynchronous state,first and second resistors connected in series between the positive sideof said rectifier bridge and the control electrode of said thyristormeans, and a diode having its anode side connected to the junctionbetween said series-connected resistors and its cathode side connectedto the anode side of said thyristor means.

2. An arrangement for exciting the field winding of asynchronous motoras defined in claim 1 wherein said rectifier bridge is composed entirelyof uncontrolled diodes and said thyristor means is constituted by athyristor connected to the uncontrolled diodes in the negative side ofthe bridge.

3. An arrangement for exciting the field winding of a synchronous motoras defined in claim 1 wherein the positive half of said rectifier bridgeis composed of uncontrolled diodes and the negative half of thyristors,the control electrodes of said thyristors being connected through saidseries-connected resistors to the positive side of said rectifierbridge.

4. In an arrangement for exciting the rotatable field windingof anasynchronous-start synchronous motor, the combination comprising anexciter having an alternating current output winding mounted on therotor shaft of said synchronous motor for rotation therewith, a resistorconnected in parallel with said field winding, a rectifier bridge of thesemiconductor type also mounted on said rotor shaft for rectifying thealternating current output of said exciter and delivering 'directcurrent tosaid parallel-connected field winding and resistor, a'thyristor connectedbetween one end of said resistor and the negativeside of said rectifier bridgeand which is poled to conduct current inthe direction toward said bridge, first and second resistors connectedin series between the positive side of said rectifier bridge and thecontrol electrode of said thyristor and a diode connected at its anodeside to the junction between said series-connected resistors and theanode side of said thyristor.

5. An arrangement for exciting the field winding of a synchronous motoras defined in claim 4 wherein said rectifier bridge is composed ofuncontrolled diodes, the cathode of said thyristor being connected tothe anode sides of the diodes in the negative half of said rectifierbridge and the anode of said thyristor being connected to thecorresponding end of said resistor which is paralleled with said fieldwinding.

6. An arrangement for exciting the field winding of a synchronous motoras defined in claim 5 wherein a plurality of said thyristors areprovided with their anode-cathode sections connected in parallel and thecathode side of said diode whose anode is connected to thejunctionbetween said resistors extends to the anodes of all of saidthyristors.

i 7. An arrangement for exciting the field winding of a synchronousmotor as defined in claim 4 and which further includes a second diodehaving its anode side connected to one end of said-second resistor andits cathode connected to the control electrode of said thyristor. v 1

8. In an arrangement for exciting the rotatable field winding of anasynchronous-start synchronous motor, the combination comprising anexciter havingan alternating current output winding mounted on the rotorshaft of said synchronous motor for rotation therewith, a resistorconnected in parallel with said field winding, a rectifier bridge of thesemiconductor type also mounted on said rotor shaft for rectifying thealternating current output of said exciter and delivering direct currentto said parallel-connected field winding and resister, the positive halfof said rectifier bridge being composed of uncontrolled diodes and thenegative half of thyristors, first and second resistors connected inseries between the positive side of said rectifier bridge and thecontrol electrodes of said thyristors, and a diode having its anode sideconnected to the junction between said series-connected resistors andits cathode side connected to the anode sides of said thyristors.

1. In an arrangement for exciting the rotatable field winding of anasynchronous-start synchronous motor, the combination comprising anexciter having an alternating current output winding mounted on therotor shaft of said synchronous motor for rotation therewith, a resistorconnector in parallel with said field winding, a rectifier bridge of thesemiconductor type also mounted on said rotor shaft for rectifying thealternating current output of said exciter and delivering direct currentto said parallel-connected field winding and resistor, thyristor meansconnected into the negative side of saId rectifier bridge forinterrupting the flow of direct current to said field winding when saidmotor is operating in its asynchronous state, first and second resistorsconnected in series between the positive side of said rectifier bridgeand the control electrode of said thyristor means, and a diode havingits anode side connected to the junction between said series-connectedresistors and its cathode side connected to the anode side of saidthyristor means.
 2. An arrangement for exciting the field winding ofasynchronous motor as defined in claim 1 wherein said rectifier bridgeis composed entirely of uncontrolled diodes and said thyristor means isconstituted by a thyristor connected to the uncontrolled diodes in thenegative side of the bridge.
 3. An arrangement for exciting the fieldwinding of a synchronous motor as defined in claim 1 wherein thepositive half of said rectifier bridge is composed of uncontrolleddiodes and the negative half of thyristors, the control electrodes ofsaid thyristors being connected through said series-connected resistorsto the positive side of said rectifier bridge.
 4. In an arrangement forexciting the rotatable field winding of an asynchronous-startsynchronous motor, the combination comprising an exciter having analternating current output winding mounted on the rotor shaft of saidsynchronous motor for rotation therewith, a resistor connected inparallel with said field winding, a rectifier bridge of thesemiconductor type also mounted on said rotor shaft for rectifying thealternating current output of said exciter and delivering direct currentto said parallel-connected field winding and resistor, a thyristorconnected between one end of said resistor and the negative side of saidrectifier bridge and which is poled to conduct current in the directiontoward said bridge, first and second resistors connected in seriesbetween the positive side of said rectifier bridge and the controlelectrode of said thyristor and a diode connected at its anode side tothe junction between said series-connected resistors and the anode sideof said thyristor.
 5. An arrangement for exciting the field winding of asynchronous motor as defined in claim 4 wherein said rectifier bridge iscomposed of uncontrolled diodes, the cathode of said thyristor beingconnected to the anode sides of the diodes in the negative half of saidrectifier bridge and the anode of said thyristor being connected to thecorresponding end of said resistor which is paralleled with said fieldwinding.
 6. An arrangement for exciting the field winding of asynchronous motor as defined in claim 5 wherein a plurality of saidthyristors are provided with their anode-cathode sections connected inparallel and the cathode side of said diode whose anode is connected tothe junction between said resistors extends to the anodes of all of saidthyristors.
 7. An arrangement for exciting the field winding of asynchronous motor as defined in claim 4 and which further includes asecond diode having its anode side connected to one end of said secondresistor and its cathode connected to the control electrode of saidthyristor.
 8. In an arrangement for exciting the rotatable field windingof an asynchronous-start synchronous motor, the combination comprisingan exciter having an alternating current output winding mounted on therotor shaft of said synchronous motor for rotation therewith, a resistorconnected in parallel with said field winding, a rectifier bridge of thesemiconductor type also mounted on said rotor shaft for rectifying thealternating current output of said exciter and delivering direct currentto said parallel-connected field winding and resistor, the positive halfof said rectifier bridge being composed of uncontrolled diodes and thenegative half of thyristors, first and second resistors connected inseries between the positive side of said rectifier bridge and thecontrol electrodes of said thyristors, and a diode having its anode sideconnected to thE junction between said series-connected resistors andits cathode side connected to the anode sides of said thyristors.